Mercury is an extremely toxic element. Numerous studies in North America and Europe have highlighted the problems of high mercury levels in food samples and the like, as well as the difficulties in the preservation of samples and the accurate and precise analysis of these.
Due to the toxic nature of mercury, even low levels can be significant. Accordingly, it is becoming increasingly desirable to measure mercury, in the amounts of parts per billion or parts per trillion, in air, water and food samples. Various techniques are known for detecting the presence of mercury. Two common techniques rely either on atomic absorption or atomic fluorescence. For atomic fluorescence, a Cold Vapour Atomic Fluorescence Spectrophotometer (CVAFS) is preferred. As compared to atomic absorption, the phenomenon is linear over a much wider range and is not subject to positive interferences.
Whatever technique is used, there is the fundamental problem of gathering a mercury sample. One well-known technique is to preconcentrate the mercury onto an adsorber. Here, it is well known that gold is an excellent adsorber for mercury, since the gold and mercury form an amalgam. Further, gold shows good properties, in terms of rejecting and not adsorbing possible contaminants, and the adsorbed mercury can be readily desorbed, simply by heating the gold. In this specification including the claims reference is made to "adsorption", which for the adsorption of mercury onto the surface of gold refers to the amalgamation of the mercury in a thin surface layer of the gold, so that there is no "absorption" into the interior of the gold. For other substances, in the specification including the claims, reference to "adsorption" is a reference to any process whereby a substance of interest is retained on the surface of a solid or in a thin surface layer immediately adjacent the surface, so as to be capable of being readily desorbed into a vapour steam.
However, when collecting minute or trace amounts of mercury, and to ensure an accurate measurement, it is necessary that the gold be in a form to provide a large surface area. Additionally, the configuration of the gold should be such as to provide an unimpeded passage for the relevant fluid through it.
One known technique relies upon the use of so-called Gold Sand. An article entitled "Monitoring Elemental Mercury in an Urban Environment" by P. B. Stockwell et al (Process Control and Quality, 1 (1991) 293-296) refers to the use of gold quartz sand to collect mercury, and makes reference to an article by R. Dumarey et al. The coated gold sand comprises quartz sand coated with a thin layer of gold. The sand is provided in a tube secured in place using quartz wool plugs. The problem with this arrangement is that the sand cracks or the gold layer flakes off, exposing chemically active sites that adsorb many interfering compounds. Microscopic sand and gold particles resulting from the breakdown can adsorb mercury and/or cause contamination of the downstream components. The quartz wool plugs cause problems since they adsorb a wide range of compounds other than mercury and release them into the detector in the heating cycle, causing erroneous results. Fine grain quartz wool will trap water under high humidity condensing conditions, which can cause numerous unwanted chemical processes to occur. Unlike quartz tubes, quartz wool presents a large number of active sites which can trap interfering compounds. This effect is particularly noticeable in cases where the ambient air contains common pollutants (e.g. H .sub.2 S, SO.sub.2, NO.sub.x), especially under high humidity conditions.
Some other known cartridges use gold coated glass beads rather than sand. These exhibit similar problems with the gold flaking and quartz wool end plugs, although the beads do not crack.
Other disclosures of the use of gold as an adsorbent can be found in U.S. Pat. Nos. 3,884,639; 4,023,929; 5,026,652.
The Sugiyama U.S. Pat. No. 3,884,639, discloses a mercury capturing zone including an amalgamating agent. It simply notes that the mercury or other amalgamating agent is a metal, such as gold, and that the metal may be used alone or may be carried on a heat-resistant support such as quartz powder.
The Becker et al U.S. Pat. No. 4,023,929 discloses a process for determining traces of mercury in liquids. An adsorption zone includes a gold wire mesh, but it gives no further detail of the structure of the wire mesh.
The Huber U.S. Pat. No. 5,026,652 discloses a method and device for mercury analysis. It simply mentions that it has a gold net for capturing the mercury.
It is to be appreciated that none of these patents discusses the detail of the mercury adsorption section of the device in any detail.
There are also numerous other fields where it is necessary to collect or preconcentrate an analyte or substance entrained in a fluid by either adsorption onto a solid surface. Two problems are almost always present, namely, (i) providing a large surface area, while maintaining a relatively low flow restriction; and (ii) maintaining an effective seal, so that a fluid cannot flow around the solid material. Feature (i) is desirable so that a large quantity of fluid can be passed through quickly to give a large sample; while feature (ii) is particularly relevant where heating and cooling cycles are used since inevitably the solid and a surrounding container have different coefficients of thermal expansion, and commonly heating is used to desorb a sample.